Free space on C:

As of lately i m runing into this problem, i m always out of space on C:
It has 40gb and after manualy checking only 21 gb are occupied by windows/programs and such. Right now it says i have 2gb free, after doing a disck cleanup (wich deleted about 1gb of stuff after i checked every box.) Where on earth are the other 17 gb??
I doubt it s in virtual memory since i have 6 gb of RAM and task manager shows 1400 mb in Use right now.
I m runing windows 8.